The present invention relates generally to electromagnetic wave transmission lines and relates more particularly to a transmission line of the three-plate air type operating at ultrahigh frequency.
Generally, it is known that a line of the three-plate air type comprises two parallel conducting plates, spaced apart from each other, and connected electrically together, the space separating these two plates being filled with air serving as dielectric, and a central conducting strip placed between the two plates and parallel thereto.
However, the usual construction of ultrahigh-frequency lines of the three-plate type makes use of a dielectric material plate arranged between the two flat conductors. Thus, one of these known three-plate lines comprises a plate forming a dielectric material support, made for example from glass-Teflon, placed between the two conducting plates and on which is disposed, for example by photo-etching, the central conducting strip. Furthermore, said support-forming plate is held in place by means of a plurality of metal posts arranged in alignment on each side of the conducting strip and mounted in twos by superimposition between said strip and the two conducting plates, respectively.
However, such a three-plate line presents drawbacks. In fact, because of the presence of the dielectric material support, this type of line is limited in length, of the order of 1 m. Furthermore, the dielectric material forming the support has a poor temperature resistance, causing consequently deformation thereof. Moreover, the metal posts with reduced distance between axes are indispensable for suppressing the evanescent modes due to the presence of the dielectric support. This line is therefore expensive, of a relatively high weight and causes high losses.